282 Darwin, and after Darwin. 
various mechanisms in question have themselves had 
a history—beginning in the forms of most uniformity 
and simplicity, gradually advancing to forms more 
varied and complex, nowhere exhibiting any inter- 
ruptions in their upward progress, until the world of 
organic machinery as we now have it is seen to have 
been but the last phase of a long and gradual growth, 
the ultimate roots of which are to be found in the soil 
of undifferentiated protoplasm. 
Lastly, when there is supplied to us the suggestion 
of natural selection as a cause presumably adequate to 
account for this continuous growth in the number, the 
intricacy, and the perfection of such mechanisms, it is 
only the most unphilosophical mind that can refuse to 
pause as between the older hypothesis of design and 
the newer hypothesis of descent. 
Thus it is clear that the a friori standing of the 
rival hypotheses of naturalism and supernaturalism in 
the case of all these pieces of organic machinery, is 
profoundly affected by the question whether they came 
into existence suddenly, or whether they did so grad- 
ually. For, if they all came into existence suddenly, 
the fact would constitute well-nigh positive proof 
in favour of supernaturalism, or creation by design ; 
whereas, if they all came into existence gradually, this 
fact would in itself constitute presumptive evidence in 
favour of naturalism, or of development by natural 
causes. And, as shown in the previous chapters, 
the proof that all species of plants and animals came 
into existence gradually—or the proof of evolution as 
a fact—is simply overwhelming. 
From a still more general point of view I may state 
the case in another way, by borrowing and somewhat 
